The multipath reception of NTSC television images, commonly referred to as "ghosting", is a widespread problem both for signals received directly off-air and for signals received from cable TV systems. Recent advances in digital signal processing technology make it both practical and economical to implement a ghost-canceling system in consumer television receivers that will eliminate, or at least substantially reduce, the deleterious effects of multipath reception.
The signal to which the television receiver synchronizes is called the reference signal, and the reference signal is usually the direct signal received over the shortest transmission path. The multipath signals received over other paths are thus usually delayed with respect to the reference signal and appear as trailing ghost images. It is possible however, that the direct or shortest path signal is not the signal to which the receiver synchronizes. Where the receiver synchronizes to a reflected (longer path) signal, there will be a leading ghost image caused by the direct signal, or there will a plurality of leading ghosts caused by the direct signal and other reflected signals of lesser delay than the reflected signal to which the receiver synchronizes. The multipath signals vary in number, amplitude and delay time from location to location and from channel to channel at a given location.
The visual effects of multipath distortion can be broadly classified in two categories: multiple images and distortion of the frequency response characteristic of the channel. Both effects occur due to the time and amplitude variations among the multipath signals arriving at the reception site. When the relative delays of the multipath signals with respect to the reference signal are sufficiently large, the visual effect is observed as multiple copies of the same image on the television display displaced horizontally from each other. These copies are sometimes referred to as "macroghosts" to distinguish them from "microghosts", which will be presently described. Usually the direct signal predominates and a receiver is synchronized to the direct signal, and the ghost images are displaced to the right at varying position, intensity and polarity. These are known as trailing ghosts or "post-ghost" images. When the receiver synchronizes to a reflected signal, there will be one or more ghost images displaced to the left of the reference image. These are known as leading ghosts or "pre-ghost" images.
Multipath signals of relatively short delay with respect to the reference signal do not cause separately discernible copies of the predominant image, but introduce distortion into the frequency response characteristic of the channel. The visual effect in this case is observed as increased or decreased sharpness of the image and in some cases loss of some image information. These short-delay or close-in ghosts are most commonly caused by unterminated or incorrectly terminated radio frequency transmission lines such as antenna lead-ins or cable television drop cables. In a cable television environment, it is possible to have multiple close-in ghosts caused by multiple distortion taps and multiple improperly terminated drop cables of varying lengths. Such multiple close-in ghosts are frequently referred to as "micro-ghosts".
The phenomenon of television ghosts has been addressed with a view to improving picture quality by reducing or eliminating ghosts. See, for example, "A Tutorial On Ghost Canceling In Television Receivers" by W. Ciciora et al., published February 1979 in the IEEE Transactions On Consumer Electronics, volume CE-25, pages 9-43. Other solutions to the problem of ghosts are described in U.S. Pat. No. 4,896,213 issued Jan. 23, 1990 to Kobo et al. and U.S. Pat. No. 4,897,725 issued Jan. 30, 1990, to Tanaka et al., the disclosures of which patents are incorporated herein by reference.
Since the characteristics of a transmitted television signal are known a priori, at least in theory it is possible to utilize such characteristics in a system of ghost signal detection and cancellation. Nevertheless, various problems limit this approach. Instead, it has been found desirable to transmit repeatedly a reference signal situated, for example, in a section of television signal that is currently unused for video purposes and to use this reference signal for detection and cancellation of ghost signals. Typically, lines in the vertical blanking interval (VBI) are used. Such a signal is herein referred to as a Ghost Canceling Reference (i.e., a "GCR") signal.
The strategy for eliminating ghosts in a television receiver relies on the transmitted GCR signal suffering the same multipath distortions as the rest of the television signal. The receiver can then examine the distorted GCR signal it receives and, with a priori knowledge of the waveform of a distortion-free GCR signal, can configure an adaptive filter to cancel, or at least significantly attenuate, the multipath distortion. It is important to choose a GCR signal that does not take up too much time in the VBI (preferably no more than one TV line), but that still contains sufficient information to permit the receiver to analyze the multipath distortion and configure an compensating filter to cancel the distortion.
It has been proposed that a useful test or GCR signal may appropriately exhibit a (sin x)/x waveform. Such a waveform, suitably windowed, exhibits a relatively constant spectral energy density over a frequency band of interest as noted in the paper by W. Ciciora et al. Ghost locations can then be determined for ghost signal cancellation to reduce the effects of long multipaths and waveform equalization to reduce the effects of short multipaths.
U.S. Pat. No. 4,896,213 to Kobo notes a ghost-canceling signal transmission/reception system which allows a built-in-ghost-canceling device to reduce or eliminate ghost components attributable to group-delay distortion and frequency-amplitude characteristic distortion generated in a signal transmission path. This is achieved by superimposing a digital signal on a television signal as a ghost-canceling reference signal. Thus, in U.S. Pat. No. 4.896,213, a digital signal composed of frame synchronizing signals, clock synchronizing signals, and data signals is generated, and is superposed, during the vertical blanking interval, on a television signal to be transmitted. At the receiving end, the digital signal superposed on the television signal is used as a reference signal in an arrangement that executes a correlative operation of the transmitted television signal to reduce the ghost phenomenon.
In the arrangement of U.S. Pat. No. 4,897,725 to Tanaka, a transmitted reference or GCR signal is also used. A dummy ghost signal is generated and is used for cancelling a ghost signal in the transmitted television signal. This is substantially the GCR signal proposal of the BTA in Japan, which uses as the main reference or deghosting signal a signal having the aforementioned (sin x)/x waveform, principally because its frequency spectral energy content is substantially flat across the entire video bandwidth. Averaging with a pair-wise constant signal is used for deriving a received reference waveform. The received reference waveform is Fourier transformed to provide a set of Fourier coefficients. The transformed reference waveform is then processed with an available fast Fourier transform (FFT) of an unimpaired GCR to compute the deghosting filter parameters, that is, tap gain information for both an infinite-impulse-response (IIR) filter (referred to as a deghosting filter) used to suppress macroghosts that occur after the predominant signal and for a finite-impulse-response (FIR) filter (referred to as a waveform-equalization filter) used to suppress microghosts.
As can be expected, the ghost cancelation reference signal is generally received accompanied by its ghost signals and is thus itself a "ghosted" signal. It is herein recognized that the performance of a ghost-canceling system is greatly influenced by the noise and perturbation content of the acquired GCR signal. It is also recognized that a reduction in the noise and perturbation content of the acquired GCR signal is desirable in improving the accuracy of the deghosting filter parameter derivations and in reducing the system complexity.
It is herein further recognized that a step in the signal leading edge is desirable in a GCR signal in computing ghost locations. As previously mentioned, a (sin x)/x waveform provides particular advantages in a GCR signal. Its flat frequency spectrum allows accurate computation of the filter parameters for attenuating multiple image effects as well as computation of the waveform equalizing parameters. The characteristic ripples of the (sin x)/x waveform however, along with other high frequency components, are typically attenuated in a received ghost GCR, owing to multipath effects as well as to to effects of antenna misorientation, each of which effects commonly occurs in practice. Under such conditions, the computation of the waveform equalizing parameters can be significantly in error.
David Sarnoff Research Center of Princeton N.J. has proposed the use of a pseudo-noise binary sequence transmitted during a VBI scan line in each frame as a GCR signal. The pseudo-noise binary sequence is transmitted on a pedestal. A corresponding pedestal, either being free of accompanying signal or having superposed thereon the same pseudo-noise binary sequence of opposite sense of polarity, can be transmitted during the corresponding VBI scan line in the other field of each frame. If this is done, differentially combining the signals transmitted during the corresponding VBI scan lines in the two fields of each frame recovers the pseudo-noise binary sequence and its ghosts without accompanying direct or color burst components or their ghosts. This cancellation occurs owing to what is referred to as "pair-wise-constant" signal processing. The pseudo-noise binary sequence and its ghosts can be correlated with the cyclically repeated ghost-free pseudo-noise binary sequence known a priori to obtain a set of pulses having strengths and spacings that characterize the relationships of the predominant pseudo-noise binary sequence and its ghosts in the time domain. This is a procedure referred to as "circular" correlation because of the cyclic nature of the correlation filter bed, each cycle of which conforms to the ghost-free pseudo-noise binary sequence. Circular correlation leads to the aliasing of ghosts that differ in time from the predominant signal by more than a cycle time, so the correlation result provides no basis for distinguishing such ghosts from ghosts differing in time from the predominant signal by less than a cycle time.
"Complementary sequences" are described by M. J. E. Golay in his paper "Complementary Series" published April 1961 in the IRE Transactions On Information Theory, volume IT-7, pages 82-87. C.-C. Tseng and C. L. Liu have further expanded the theory of complementary sequences in their paper "Complementary Sets of Sequences" published in September 1972 in the IEEE Transactions On Information Theory, volume IT-18, pages 644-652. By definition, a pair of binary (.+-.1) sequences of similar length of each other are complementary sequences only if the sum of the linear autocorrelation functions of the sequences is identically zero for all shifts other than zero, and provides a high correlation gain at zero shift. In linear correlation the correlation filter bed conforms to the sampled function against which correlation is made appearing a single time, being preceded by an "infinite" string of zeroes and being succeeded by an "infinite" string of zeroes.
Unlike ordinary pseudo-random binary sequences which exist only for lengths N=2.sup.n -1, complementary sequences have a less stringent constraint on their lengths. Unfortunately, there exists no general method of finding complementary-sequence pairs for arbitrary lengths, and an exhaustive computer search quickly becomes impractical as the length increases. However, a property of complementary sequences proven by Tseng and Liu offers an efficient technique to synthesize longer length complementary-sequence pairs E and F from two known shorter length pairs (A,B) of length m and (C,D) of length n. The successive bits in sequence A are a.sub.1,a.sub.2. . . a.sub.(m-1),a.sub.m. The successive bits in sequence B are b.sub.1,b.sub.2, . . . b.sub.(m-1),b.sub.m. The successive bits in sequence C are c.sub.1, c.sub.2, . . . c.sub.(n-1),c.sub.n. The successive bits in sequence D are d.sub.1,d.sub.2, . . . d.sub.(n-1),d.sub.n.
The successive bits in sequence E are EQU (a.sub.1 c.sub.1), . . . (a.sub.m c.sub.1),(b.sub.1 d.sub.1), . . . (b.sub.m d.sub.1), . . . (a.sub.1 c.sub.n) . . . (a.sub.m c.sub.n),(b.sub.1 d.sub.n), . . . (b.sub.m d.sub.n);
and the successive bits in sequence F are EQU (a.sub.1 d.sub.n), . . . (a.sub.m d.sub.n),(-b.sub.1 c.sub.n), . . . (-b.sub.m c.sub.n), . . . (a.sub.1 d.sub.1) . . . (a.sub.m d.sub.1),(-b.sub.1 c.sub.1), . . . (-b.sub.m c.sub.1) (1)
where the bit xy=x if y=1 and xy=-x if y=-1.
Thus a length 2 mm complementary-sequence pair can be synthesized if length m and length n complementary-sequence pairs are known. By a computer search, primitive, complementary-sequence pairs of length 2, 4, 8, 10 are easily found. Thus, using equation (1), one can obtain complementary-sequence pairs of extended length 2.sup.m 10.sup.n, where m and n are integers. Complementary sequence pairs of extended length 640, where m is 6 and n is 1, can be obtained, by way of example.
Another way to generate longer-length, ternary complementary-sequence pairs, the inventors find, is to expand shorter-length binary complementary-sequence pairs by inserting a prescribed number of null samples (zeros) between each +1 or -1 sample and the next +1 or -1 sample. The longer-length, ternary complementary-sequence pair results from subsampling and is therefore of reduced spatial bandwidth. Linear correlation filtering with the reduced spatial bandwidth. Linear correlation filtering with the reduced bandwidth kernel discards higher-spatial-frequency noise, resulting in somewhat improved signal-to-noise correlation filter responses obtaining when using the ternary complementary-sequence pair.